1. Field of the Invention
The present disclosure is a method for synthesizing bio-plasticizers using acidic ionic liquids as catalysts, particularly a method to epoxidize fatty acid alkyl esters by using Bronsted acidic ionic liquids as catalysts.
2. Brief Description of the Prior Art
The plastic additive is an imperative annexing agent for plastic processing, and is also known as plasticizer, plastifiant or elasticizer for more flexibility of a material. Plasticizers common in the industry are phthalate ester plasticizers which account for 85% of total output. For more flexibility and viscosity, phthalate ester plasticizers can be added into PVC for manufacturing plastic wraps, tubes and coating materials on electric wires or cables. However, phthalate esters with short carbon chains are totally petrochemical products and both phthalate ester plasticizers and their metabolites in human bodies are environmental hormones with adverse effects on metabolism and reproductive function. To decrease carbon footprints and enhance sustainable development of the environment, more and more manufacturers have been devoting themselves to developing environment-friendly plasticizers as well as bio-plasticizers.
Based on raw material of vegetable oil, epoxidized vegetable-oil plasticizers with good stability are popular bio-plasticizers, but are still mixed with phthalate ester plasticizers in applications because of the defect of poor compatibility. Comparably, epoxidized fatty acid alkyl esters can be taken as substitutes of conventional plasticizers, without the drawbacks of epoxidized vegetable oil. Currently, epoxidized plasticizers are usually manufactured in a process in which epoxy groups are generated by peroxy acid and unsaturated double bonds with concentrated sulfuric acid added as a catalyst. However, final products must be rinsed with a great quantity of water because concentrated sulfuric acid as a catalyst cannot be removed easily after reactions. Moreover, a great number of liquid waste to be discharged have to be neutralized in advance, but can still cause problems such as corrosion in equipment or pipelines.
To solve the above-noted problems related to use of concentrated sulfuric acid, a method which is based on reactants of formic acid, without strong-acid catalysts, induces autocatalysis which lasts for a long period (Patent No. CN 100590188C). Sobczak et al. used transition-metal compounds as catalysts, for example, molybdena catalyst/cumene hydroperoxide (CHP) or molybdena catalyst/tert-butyl hydroperoxide (t-BuOOH) composite catalyst system, and unsaturated fatty acid in reactions at 80° C. for 140˜280 minutes to epoxidize oleic acid successfully wherein selectivity of epoxidation on the basis of the Mo(O)2(SAP)(EtOH)/t-BuOOH system is 86.8% (J. M. Scobczak, J. J. Ziólkowski, Appl. Catal. A, 248 (2003) 261-268). Farias et al. employed the MoO2(acac)2/t-BuOOH catalyst system to epoxidize soybean oil in 2-hour reactions at 110° C. and derived selectivity of epoxidation of 77.2% (M. Farias, M. Martinelli, D. P. Bottega, Appl. Catal. A, 384 (2010) 213-219). There have been many researchers in the academic community or the industry developing manufacturing processes which depend on solid catalysts to epoxidize vegetable oil or Fatty Acid Methyl Ester (FAME), for example, solid acidic catalysts (Patent No. CN 102489333A; CN 103113993A; CN 104326911A), solid base catalysts (Patent No. CN 101029177A), composite sulfate catalysts (Patent No. CN 102060812A), magnetic solid acidic catalysts (Patent No. CN 101885710A), because of some advantages including less acidic liquid waste in epoxidation based on solid catalysts and easy separations between heterogeneous catalysts. However, some problems such as activity decline of solid catalysts and reactivation of solid catalysts to be reused are still unavoidable.
To correct the above drawbacks, researchers employed ionic liquids with advantages such as easy separation, reusability, less corrosion in pipelines, and properties of both solid catalysts and traditional liquid acids to catalyze epoxidation. Chen et al. synthesized epoxidized fatty acid esters and hydroxylated fatty acid esters by ionic liquids of sulfoalkyl imidazolium salt as catalysts, mixing epoxidized fatty acid esters, hydroxylated fatty acid esters and additives to derive stamping and drawing oil (Patent No. CN 102787007B). Shen et al. employed 1-dodecyl-3-methylimidazolium bromide and ammonium perrhenate (NH4ReO4) for preparation of hydrophobic rhenium ionic liquids which were taken as catalysts to derive epoxidized FAME (epoxy value=2.65˜4.57) (Patent No. CN 104327015A). Shen et al. used ionic liquids with sulfonic groups and heteropoly acids as catalysts to derive epoxidized FAME and continuously added glycerin in reactions to synthesize bio-oil-based polyol (Patent No. CN 104341297A). Chen et al. chose tetrafluoroborate, alkyl quaternary ammonium and alkyl quaternary phosphonium (alkyl replacing pyridinium and imidazolium) to prepare ionic liquids which were further taken as catalysts to synthesize expoxidized fatty acid methyl esters (epoxy value=3.1˜4.7) (Patent No. CN 101284821A).